SwiftQUIC: Fix variance in Spin Bit testing

Author: agnosticdev

Sources/SwiftNetwork/QUIC/QUICConnection.swift

@@ -2322,6 +2322,9 @@ public final class QUICConnection: ManyToManyApplicationStreamProtocol,
                     // Reflect the spin bit.
                     let spinValue = packet.spinValue
                     path.spinValue = spinValue
+                    if spinValue {
+                        path.spinValueForTestingSeen = true
+                    }
                 } else {
                     // Spin it!
                     let spinValue = packet.spinValue

Sources/SwiftNetwork/QUIC/QUICPath.swift

@@ -188,6 +188,7 @@ public final class QUICPath: MultiplexingDatagramPath<QUICConnection>, Equatable
         static let isFlowControlled = Flags(rawValue: 1 << 9)
         static let l4sEnabled = Flags(rawValue: 1 << 10)
         static let reportedIdleEvent = Flags(rawValue: 1 << 11)
+        static let spinValueForTestingSeen = Flags(rawValue: 1 << 12)
     }
     private var flags = Flags()
 
@@ -241,6 +242,10 @@ public final class QUICPath: MultiplexingDatagramPath<QUICConnection>, Equatable
         get { flags.contains(.reportedIdleEvent) }
         set { if newValue { flags.insert(.reportedIdleEvent) } else { flags.remove(.reportedIdleEvent) } }
     }
+    var spinValueForTestingSeen: Bool {
+        get { flags.contains(.spinValueForTestingSeen) }
+        set { if newValue { flags.insert(.spinValueForTestingSeen) } else { flags.remove(.spinValueForTestingSeen) } }
+    }
 
     var log: LogPrefixer {
         parentProtocol.logPrefixer

Tests/SwiftNetworkTests/SwiftNetworkQUICSpinBitTests.swift

@@ -91,7 +91,10 @@ final class SwiftNetworkQUICSpinBitTests: NetTestCase {
                     defer { expectation.fulfill() }
 
                     let clientSpinBit = harness.state?.clientInstance.currentPath?.spinValue ?? false
-                    let serverSpinBit = harness.state?.serverInstance.currentPath?.spinValue ?? false
+                    // Note that spinValueForTestingSeen is checked here because it could be that by the time
+                    // this test checks the server spin bit value it has received the ACK from the client and the
+                    // spin bit value has already changed. This reliably make sure that the spin bit on the server was present.
+                    let serverSpinBit = harness.state?.serverInstance.currentPath?.spinValueForTestingSeen ?? false
                     XCTAssertFalse(clientSpinBit, "Client should have the spin bit set to false")
                     XCTAssertTrue(serverSpinBit, "Server should have the spin bit set to true")
                 }